Various types of titanium alloy having high-grade mechanical characteristics are known that include a significant proportion of aluminum, such as for example Ti 6-4 (6% aluminum and 4% vanadium), Ti 8-1-1 (8% aluminum, 1% molybdenum, and 1% vanadium), and also Ti 10-2-3 (10% vanadium, 2% iron, and 3% aluminum), where the percentages represent a proportion by weight relative to the total weight. Titanium alloys are also known that are of the quasi-beta type, having a large proportion of aluminum and also of oxygen. An example of such an alloy is given by document EP 1 302 555 that describes a titanium alloy presenting the following composition, expressed as percentages of total weight:
Aluminum4.0 to 6.0Vanadium4.5 to 6.0Molybdenum4.5 to 6.0Chromium2.0 to 3.6Iron0.2 to 0.5Zirconium0.7 to 2.0Oxygennot more than 0.2Nitrogennot more than 0.05Titaniumbalance
Such alloys are for hot forging at a temperature that is close to the β→α+β polymorphic transition temperature, and then for subjecting to heat treatment during which the part is heated to a temperature close to the β→α+β polymorphic transition temperature in order to cause a beta phase to appear that coexists with an alpha phase, followed by staged cooling and aging of the part. The purpose of such treatment is to obtain a large proportion of beta phase in the finished part, so as to give it great mechanical strength. In this respect, elements, such as vanadium, molybdenum, chromium, or iron contribute to stabilizing the beta phase while the part is cooling, thus making it possible to “freeze” a large portion of the alloy in this phase.
Nevertheless, promoting the beta phase generally takes place to the detriment of the alpha phase (typically representing 60% to 70% of the weight of a part made in this alloy), which alpha phase enhances the toughness of the part. In order to mitigate that drawback, a non-negligible proportion of zirconium is added to the composition in order to enhance alpha phase stabilization during cooling, by forming solid solutions with the alpha titanium, with which zirconium is relatively similar in terms of density and melting temperature.
The use of such a composition and the implementation of appropriate forging and heat treatment methods (in particular cooling that encourages the above-mentioned solid solution) enable solid titanium parts to be produced that present an advantageous compromise between toughness and mechanical strength.